The present invention relates to a method for reducing short time-scale reciprocity failure effects of a microencapsulated acrylate system.
Reciprocity failure may be exhibited by photographic materials. For a photographic system to obey reciprocity, the photographic system must produce the same image density with either an exposure to a high intensity light for a short exposure time or an exposure to a low intensity light for a long exposure time, providing that the total exposure energy is the same in both cases. If the photographic material fails to behave in this manner, the photographic material is said to suffer from "reciprocity failure."
Short time-scale reciprocity failure occurs in imaging systems such as those disclosed in commonly assigned U.S. Pat. Nos. 4,399,209 and 4,440,846. In these imaging systems, the imaging sheet comprises a support with a layer of microcapsules on the surface thereof. The microcapsules have an internal phase of a photohardenable photosensitive composition and color former. The microcapsules are image-wise exposed to actinic radiation and ruptured so that the color former reacts with a developer material to produce an image.
The photohardenable photosensitive composition can be described as having a viscosity which changes upon exposure to actinic radiation such that upon exposure, a change in the viscosity of the internal phase in the exposed areas occurs which image-wise determines whether the color former is accessible to the developer material. Typically, the photohardenable photosensitive composition is a radiation curable composition which, upon exposure to light, increases in viscosity and immobilizes the color former therein to prevent the color former from transferring to the developer sheet and reacting with the developer material entirely. As such, a latent image is formed in the unexposed microcapsules because upon microcapsule rupture, the color former is transferred to the developer sheet where it reacts with the developer material entirely.
In the transfer imaging system of U.S. Pat. No. 4,399,209, the imaging sheet is contacted with a developer sheet comprising a support with a layer of developer material on the surface thereof prior to microcapsule rupture. In the self-contained imaging system of U.S. Pat. No. 4,440,846, the imaging sheet has developer material co-deposited with the microcapsules on the surface thereof.
In Arney, "Oxidation Kinetics and Reciprocity Behavior in the Microencapsulated Acrylate Imaging Process," J. of Imaging Science, 31, 27 (1987), the short time-scale reciprocity failure effects of the aforementioned imaging systems were reported. According to Arney, the aforementioned microencapsulated acrylate system obeys reciprocity over the time range of about 10.sup.-2 to 10 seconds. In other words, the total exposure energy required to produce the same image density, whether by an exposure to a high intensity light for 10.sup.-2 second, an exposure to a low intensity light for 10 seconds, or an exposure in between, remains constant over this time range.
Below about 10.sup.-2 second, the total exposure energy required to produce the same image density as generated in the 10.sup.-2 to 10 seconds range is much higher. Otherwise, a loss of media sensitivity is observed. As such, the microencapsulated acrylate system is said to suffer from "short time-scale reciprocity failure." Thus, very high intensity light is required at low exposure times, i.e., below 10.sup.-2 second, in order to generate the higher exposure energy required to achieve the same image density as in the 10.sup.-2 to 10 seconds time range. It is believed that short time-scale reciprocity failure is due to a decrease in the oxidation efficiency resulting from competing processes of free radical termination.
Situations exist where short time exposure of photosensitive materials is desirable. As an example of the use of a short time exposure, see U.S. Pat. No. 4,314,256. Thus, the need exists in the art for a method for reducing the short time-scale reciprocity failure effects of the microencapsulated acrylate imaging material.